1. Field of the Invention
The present invention relates generally to nuclear fuel assemblies for nuclear reactors and, more particularly, is concerned with a nuclear fuel rod grid having unique fuel rod engaging spring structures.
2. Description of the Prior Art
In most nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies. Conventional designs of these fuel assemblies include a plurality of fuel rods held in an organized array by a plurality of grids spaced axially along the fuel assembly length and attached to a plurality of elongated control guide thimbles of the fuel assembly. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles which extend slightly above and below the ends of the fuel rods.
The grids of the fuel assembly as well known in the art are used to precisely maintain the spacing between the nuclear fuel rods in the reactor core, prevent rod vibration, provide lateral support for the fuel rods, and, to some extent, frictionally clamp and retain the rods against longitudinal movement.
One popular conventional fuel rod support grid design, being illustrated and described in U.S. Pat. No. 4,492,844 to Kobuck et al, includes a multiplicity of interleaved inner and outer belt-shaped straps having an egg-crate configuration forming a multiplicity of cells which individually accept the nuclear fuel rods and control rod guide thimbles. The cells of each grid which accept and support the fuel rods at a given axial location therealong typically use relatively resilient springs and/or relatively rigid protrusions (called dimples) formed directly into the metal of the interleaved straps. The springs and dimples of each grid cell frictionally engage or contact the respective fuel rod extending through the cell and together apply a clamping force directed generally perpendicular to the axial direction of the fuel rod. Additionally, the outer straps are attached together and peripherally enclose the inner straps to impart strength and rigidity to the grid.
However, several drawbacks arise from this particular grid design. The vertical orientation and configuration of the fuel rod engaging springs require that the springs and thus the grid be of substantial height in order for the springs to have the desired amount of resiliency and elastic deformation to function properly. There are limits to the amount by which the height of the springs and the grid can be increased to increase the elastic deformation. Furthermore, the most common material used for the support grid is Zircaloy which has a small absorption cross section for thermal neutrons but also has low elastic strength. However, increasing the grid height to increase spring height and thereby its elastic deformation, not only increases the amount of parasitic structural material utilized in the fuel assembly but also requires increase in thickness of the grid straps in order to increase rigidity from the point of view of mechanical strength. The flow path of the cooling water is then narrowed by the amount of the increase in thickness, resulting in an increase in pressure drop through the fuel assembly.
Representative of other prior art grid designs are the ones described and illustrated in U.S. patents to Kooistra (3,070,534), Wachter et al (3,928,131) and Piepers et al (3,646,994); French Patent No. 1,497,083; West German Patent No. 1,961,035; and Japanese Patent No. 61-90085. While all of these grids may function satisfactorily and generally achieve the objectives for which they were designed, none would appear to suggest an approach which will satisfactorily overcome the aforementioned drawbacks of the one conventional grid design briefly described above. Consequently, a need still exists for an improved grid spring structure which will avoid the above drawbacks without presenting new ones in their place.